1. Field of the Invention
The present invention relates to an audio converting device that does not produce noise when adjusting the gain of an audio amplifier in a digital audio system.
2. Description of the Related Art
Generally, a digital audio system can convert digital data to analog signals and play them through analog devices, such as loudspeakers. Furthermore, the digital audio system can also convert analog signals to digital data for transmission or storage. Such a digital audio system is called an Audio Codec. FIG. 1 shows a block diagram of a typical Audio Codec 60. In FIG. 1, a digital-to-analog converter (DAC) 20 converts digital data to analog signals through a digital interface 10. An analog mixing and gain control unit 30 adjusts the amplitudes of the analog signals, thereby adjusting volume. The analog mixing and gain control unit 30 can also mix the analog signals from several stereo or mono sources. The analog mixing and gain control unit 30 adjusts the amplitudes of the analog signals from each source and mixes them to be in condition for output. In addition, the analog mixing and gain control unit 30 is connected to an analog-to-digital converter (ADC) 40. Then, the analog signals are converted to digital data for further process through the digital interface 10.
In the Audio Codec, ADC and DAC can be implemented by several conventional technologies. FIG. 2 shows a typical block diagram of a DAC structure in an Audio Codec based on a delta-sigma modulation. Digital audio data DS10 are upsampled by an expander 100 and, then, fed into digital delta-sigma modulator 300 through a digital low-pass filter (digital LPF) 200. The digital delta-sigma modulator 300 then outputs one-bit stream DS40. A one-bit digital-to-analog converter (DAC) 400 converts the bit stream DS40 to analog signals AS50. Then, an analog low-pass filter 500 filters out the high-frequency noise in the analog signals AS50 and outputs filtered analog signals AS60.
For most audio systems, the volume, or the amplitude of the filtered analog signals, is adjustable by users. For example, the volume can be increased/decreased by every 0.5 dB within the range of 32 dB. FIG. 3 shows a diagram of a controllable amplifier in the analog mixing and gain control unit 30, wherein the variable-resistor Rt might have discrete values and is used to select different gains of the amplifier when users adjust the volume.
However, if the filtered analog signals AS60 inputted to the analog mixing and gain control unit 30 have a nonzero direct-current value, the output of the controllable amplifier may vary steeply as long as that the variable resistor Rt is adjusted steeply (every 0.5 dB) by users. As shown in FIG. 4, the filtered analog signals are amplified at time t with a different gain according to the variable resistor Rt due to adjustment by users. Due to the adjustment of the gain and the DC value in the analog signals, the waveform of analog outputs has a step height occurring at time t as shown in FIG. 4. This occurrence of the step height means high frequency noise in the output. The user, as a result, will hear unpleasant noise from connected loudspeakers. Further, the larger the direct-current value and the gain variation, the louder the noise is.